the scale of the molecular world

Molecular biology happens at a scale our senses cannot reach, so it pays to build some intuition for how small and how long these things are. The working ruler is the nanometre, written nm, which is one billionth of a metre, a million times smaller than a millimetre. The molecules of life are measured in nanometres, and the lengths of their information chains are measured in numbers of letters.

A few anchor figures make the world concrete. The DNA double helix is about 2 nm wide, and one step along it, one base pair, is about 0.34 nm long. A typical protein is a few nanometres across, a bacterium like E. coli about 1000 nm (one micrometre) long, and an animal cell perhaps ten to a hundred times bigger than that. For information we count in base pairs (bp) for DNA, often in thousands (kilobases, kb) or millions (megabases, Mb): a typical gene is a few kb, a bacterial genome a few Mb, and the human genome about 3000 Mb, roughly 3 billion base pairs. Stretched end to end, the DNA in a single human cell is about two metres long, yet it folds into a nucleus only about 6000 nm wide.

These numbers are not trivia; they shape what is possible. Because a base pair is only a third of a nanometre, an enormous amount of information packs into a tiny space, which is why a cell must solve a serious packaging problem to fit two metres of DNA into a microscopic nucleus. The same scale explains why we need indirect tools, copying with PCR, sequencing, and powerful microscopes, to see what is going on, since the players are far too small for ordinary light to resolve.